Particulate counters utilize the principle of near angle optical scattering to permit the counting and sizing of small particles contained within a sealed vessel. A portion of a light beam projected into the vessel is deflected by the presence of various moving particles within the beam. The scattered portion of the beam may be focused onto a photodiode and may be quantified so that a statistical analysis of the photodiode output waveform may be performed to yield an estimate of the distribution, number and size of the particles within the vessel.
Exemplary of particulate counters which are known in the prior art are those disclosed by Ogle in U.S. Pat. No. 3,858,851 and by Knollenberg in U.S. Pat. No. 3,956,616, both of which are incorporated herein by reference. Such prior art particulate counters determine the validity of individual particle detections by analyzing the amplitude and duration of each discrete pulse of the photodiode output waveform. Since it is assumed in the prior art that each pulse corresponds to a single detected particle, detection of a pulse which has an amplitude less than a threshhold amplitude or which has a pulse duration less than a threshhold duration indicates that only a partial detection has occurred. Similarly, detection of a pulse having a duration in excess of a maximum duration indicates that a particle out of the desired field of focus has been detected.
In accordance with the illustrated preferred embodiment of the present invention, a particulate counter selects as a valid particle detection only a single maximum photodiode waveform peak within a time window or sample period. Because of interference occurring between the main beam and the portion of the beam which is deflected by a given particle, the photodiode waveform caused by a valid particle detection may not be characterized as a single triangular pulse as is done in the prior art. Rather, the waveform may more accurately be characterized as a series of lobes contained within a pulse envelope. Thus, by counting each lobe as a discrete particle the prior art particle counters obtain an erroneous particle count. The particulate counter which is constructed in accordance with the illustrated preferred embodiment of the present invention selects only the single maximum peak within a time window or sample period as a valid particle detection and disregards the remaining lobes within the window. The durtion of the window may be chosen as the maximum time required for the relative movement of the beam across a stationary particle in order that multiple detections of the same particle may be disregarded.